I worked for Bosch in the early 70s. We also distributed Blaupunkt products. Car Stereo was still a very new thing. Compared with today's standard, sound quality then was abysmal. When we installed a stereo radio, usually a Blaupunkt Frankfurt model, in W111 coupes, the rear speaker would be mounted in the middle of the rear shelf with a cutout in the shelf to lower the profile. The speaker cover was mid-grey plastic, sort of battleship grey, in an elongated lozenge shape. We would re-use the front speaker and balance would be controlled with an under dash mounted fader switch. I tried finding an image on the internet, but no luck.